1. Field of the Invention
The present invention relates to semiconductor wafer cleaning and drying and, more particularly, to apparatuses and techniques for more efficiently removing fluids from wafer surfaces of interest while reducing contamination and decreasing wafer cleaning cost.
2. Description of the Related Art
In the semiconductor chip fabrication process, it is well known that there is a need to clean and dry a wafer where a fabrication operation has been performed that leaves unwanted residues on the surfaces of wafers. Examples of such a fabrication operation include plasma etching (e.g., tungsten etch back (WEB)) and chemical mechanical polishing (CMP). In CMP, a wafer is placed in a holder which pushes a wafer surface against a rolling conveyor belt. This conveyor belt uses a slurry which consists of chemicals and abrasive materials to cause the polishing. Unfortunately, this process tends to leave an accumulation of slurry particles and residues at the wafer surface. If left on the wafer, the unwanted residual material and particles may cause, among other things, defects such as scratches on the wafer surface and inappropriate interactions between metallization features. In some cases, such defects may cause devices on the wafer to become inoperable. In order to avoid the undue costs of discarding wafers having inoperable devices, it is therefore necessary to clean the wafer adequately yet efficiently after fabrication operations that leave unwanted residues.
After a wafer has been wet cleaned, the wafer must be dried effectively to prevent water or cleaning fluid remnants from leaving residues on the wafer. If the cleaning fluid on the wafer surface is allowed to evaporate, as usually happens when droplets form, residues or contaminants previously dissolved in the cleaning fluid will remain on the wafer surface after evaporation (e.g., and form spots). To prevent evaporation from taking place, the cleaning fluid must be removed as quickly as possible without the formation of droplets on the wafer surface. In an attempt to accomplish this, one of several different drying techniques are employed such as spin drying, IPA drying, or Marangoni drying. All of these drying techniques utilize some form of a moving liquid/gas interface on a wafer surface which, if properly maintained, results in drying of a wafer surface without the formation of droplets. Unfortunately, if the moving liquid/gas interface breaks down, as often happens with all of the aforementioned drying methods, droplets form and evaporation occurs resulting in contaminants being left on the wafer surface.
The most prevalent drying technique used today is spin rinse drying (SRD). FIG. 1 illustrates movement of cleaning fluids on a wafer 10 during an SRD drying process. In this drying process, a wet wafer is rotated at a high rate by rotation 14. In SRD, by use of centrifugal force, the water or cleaning fluid used to clean the wafer is pulled from the center of the wafer to the outside of the wafer and finally off of the wafer as shown by fluid directional arrows 16. As the cleaning fluid is being pulled off of the wafer, a moving liquid/gas interface 12 is created at the center of the wafer and moves to the outside of the wafer (i.e., the circle produced by the moving liquid/gas interface 12 gets larger) as the drying process progresses. In the example of FIG. 1, the inside area of the circle formed by the moving liquid/gas interface 12 is free from the fluid and the outside area of the circle formed by the moving liquid/gas interface 12 is the cleaning fluid. Therefore, as the drying process continues, the section inside (the dry area) of the moving liquid/gas interface 12 increases while the area (the wet area) outside of the moving liquid/gas interface 12 decreases. As stated previously, if the moving liquid/gas interface 12 breaks down, droplets of the cleaning fluid form on the wafer and contamination may occur due to evaporation of the droplets. As such, it is imperative that droplet formation and the subsequent evaporation be limited to keep contaminants off of the wafer surface. Unfortunately, the present drying methods are only partially successful at the prevention of moving liquid interface breakdown.
In addition, the SRD process has difficulties with drying wafer surfaces that are hydrophobic. Hydrophobic wafer surfaces can be difficult to dry because such surfaces repel water and water based (aqueous) cleaning solutions. Therefore, as the drying process continues and the cleaning fluid is pulled away from the wafer surface, the remaining cleaning fluid (if aqueous based) will be repelled by the wafer surface. As a result, the aqueous cleaning fluid will want the least amount of area to be in contact with the hydrophobic wafer surface. Additionally, the aqueous cleaning solution tends cling to itself as a result of surface tension (i.e., as a result of molecular hydrogen bonding). Therefore, because of the hydrophobic interactions and the surface tension, balls (or droplets) of aqueous cleaning fluid forms in an uncontrolled manner on the hydrophobic wafer surface. This formation of droplets results in the harmful evaporation and the contamination discussed previously. The limitations of the SRD are particularly severe at the center of the wafer, where centrifugal force acting on the droplets is the smallest. Consequently, although the SRD process is presently the most common way of wafer drying, this method can have difficulties reducing formation of cleaning fluid droplets on the wafer surface especially when used on hydrophobic wafer surfaces.
Therefore, there is a need for a method and an apparatus that avoids the prior art by allowing quick and efficient cleaning and drying of a semiconductor wafer, but at the same time reducing the formation of water or cleaning fluid droplets which may cause contamination to deposit on the wafer surface. Such deposits as often occurs today reduce the yield of acceptable wafers and increase the cost of manufacturing semiconductor wafers.
Broadly speaking, the present invention fills these needs by providing a cleaning and drying apparatus and method that removes fluids from wafer surfaces quickly while at the same time reducing droplet formation that can cause wafer contamination. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, a wafer cleaner and dryer to be used in wafer manufacturing operations is disclosed. The wafer cleaner and dryer has a proximity head which moves toward a wafer surface to complete either a cleaning or a drying operation. The proximity head includes a plurality of source inlets where the plurality of source inlets generates a first pressure on a fluid film present on the wafer surface when the proximity head is in a first position that is close to the wafer surface. The proximity head also contains a plurality of source outlets which introduces a second pressure on the fluid film present on the wafer surface when the proximity head is in the first position. The first pressure generated by the plurality of source inlets is greater than the second pressure created by the plurality of source outlets so as to create a pressure difference where the pressure difference causes removal of the fluid film from the wafer surface.
In another embodiment, a method for cleaning and drying a semiconductor wafer is disclosed. The method provides a proximity head which includes a plurality of source inlets and a plurality of source outlets. The proximity head is moved toward a wafer surface after which a first pressure is generated on a fluid film present on the wafer surface when the proximity head is in a first position that is close to the wafer surface. A second pressure is also introduced on the fluid film present on the wafer surface when the proximity head is in the first position. A pressure difference is generated where the first pressure is greater than the second pressure which causes the removal of the fluid film from the wafer surface.
The advantages of the present invention are numerous. Most notably, by creating apparatuses and methods to efficiently dry and clean a semiconductor wafer while reducing formation of droplets, wafer processing and production may be increased and higher wafer yields may be achieved due to the lower levels of contamination of the wafers. The present invention allows the improved drying and cleaning because of the use of a combination of capillary action (such as, for example, capillary attraction), suction, and gas or liquid input. The pressures generated on fluid at the wafer surface by the aforementioned forces allow optimal removal of fluid at the wafer surface with a significant reduction in droplet formation as compared with other cleaning and drying techniques. The present invention reduces the breakdown of liquid/gas interfaces and especially works well when used to process hydrophobic wafers. Therefore, the present invention evacuates fluid from wafer surfaces with extreme effectiveness while substantially reducing contaminant formation due to droplet formation.